1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof, and more particularly, to an image forming apparatus and a control method thereof capable of precisely measuring electrical characteristics changes of factors relating to developing conditions of a developing apparatus and applying the developing conditions required for stably maintaining a printing quality to the developing apparatus.
2. Description of the Related Art
Printers, such as an image forming apparatus, may be mainly classified into an inkjet printer and an electrostatic latent image printer.
The electrostatic latent image printer is provided with a photo-sensitive body, a photo-scanning device, a developing device, and a transfer device.
The developing device of the electrostatic latent image printer has developing rollers mounted in a predetermined space around the photo-sensitive body, and a developing agent-supplying device capable of supplying a developing agent through the space between the photo-sensitive body and the developing rollers during rotations of the developing rollers.
In the developing device, it is important to uniformly supply the developing agent to the photo-sensitive body from the developing rollers in order to maintain a print quality. An AC voltage is generally applied between the developing rollers and the photo-sensitive body in order to smoothly supply the developing agent from the developing rollers to the photo-sensitive body. However, a supply amount of the developing agent to the photo-sensitive body may vary according to changes of the space between the developing rollers and the photo-sensitive body (hereinafter referred to as a “developing gap”) in response to an AC voltage applied between the developing rollers and the photo-sensitive body.
U.S. Pat. No. 5,521,683 discloses a printer applying variable AC voltages to the developing rollers based on such changes to the developing gap. However, the printer disclosed in the U.S. Pat. No. 5,521,683 uses only the developing gap as a factor affecting developing currents. Namely, the printer applies a test voltage to the developing rollers and then measures a developing current flowing from the developing rollers to the photo-sensitive body, calculates from a lookup table the developing gap corresponding to the measured developing current, and determines a driving bias to be applied to the respective developing rollers in accordance with the developing gap. However, in addition to the developing gap, a resistance value may be a factor affecting the developing current flowing to the photo-sensitive body from the developing rollers. Since the resistance value of the developing rollers generally varies according to temperature and humidity changes, data on the developing gap obtained with only an effective value of the developing current may not precisely control the driving bias to be supplied to the developing rollers. As a result, a problem of difficulties in optimizing developing conditions occurs. In particular, the changes of the resistance value of the developing rollers affect the developing current more than the developing gap.